The current energy crisis and the ever-present need for greater efficiency and economy in the carpet manufacturing industry have led this industry increasingly away from the use of water soluble foam latex as a cushion backing for carpet and other similar substrates. While foam latex forms a very satisfactory cushion backing for carpet, its processing consumes enormous quantities of energy necessary to drive the water out of the latex composition in the curing thereof. Furthermore, the traditional latex process is slow, with production rates in the range of 8 to 35 linear feet per minute being the best attainable rates. Such production rates are not economically satisfactory.
In view of the above, the carpet industry has turned increasingly to blown polyurethane as a cushion backing for carpet, with the reactant composition applied directly to the primary carpet backing in a continuous type process. Two types of polyurethane systems are known in the art. In one type, the polyurethane composition is mechanically foamed, while in the other type it is chemically foamed in situ. Once a chemical foam has been established, it is not possible to vary the foam density or thickness as it is with mechanical foaming systems, unless the basic constituents of the reactive composition are changed. Thus, the chemical foaming process lacks flexibility.
A further drawback in the use of blown polyurethane as a cushion backing for carpet is the great difficulty of providing surface smoothness and a uniform thickness to the blown polyurethane backing. Very minute stitching irregularities in the primary carpet backing can be magnified as much as thirty times in the blown backing layer. To overcome this, the industry has attempted to slow down the blowing and curing process so that doctoring and other surface leveling techniques can be carried out. Such practices have not been entirely successful. These prior art processes involved long periods of dwell time which resulted in a time consuming or slow production rate.
A further difficulty in the prior art arises as a result of a tendency for the polyurethane reactive mixture applicators and mixing devices to clog up to such an extent that they are rendered almost useless. If the reactive mixture is allowed to cure or harden in such applicator equipment, it is almost impossible to restore the equipment to a proper operating condition.
It is therefore the objective of this invention to alleviate all of the above and other deficiencies of the prior art relative to producing blown polyurethane backings directly on carpet or other like substrates. In general, the present invention overcomes the problems of the prior art by employing an improved formulation for the reactive polyurethane mixture and by utilizing an essentially non-clogging high pressure airless spray applicator or other suitable means of application for the mixture which has built-in flushing means and a small internal mixing chamber for the two or more components of the reactive mixture.
The essence or heart of the present invention which completely solves the most troublesome difficulty of the prior art in achieving thickness uniformity in the directly applied polyurethane backing layer is the following. At or near the end of the blowing cycle of the reactive composition and/or early in the curing cycle while the applied backing layer possesses substantial residual tackiness, it is subjected to compressive regauging of its thickness by the application thereto of a heavy mechanical compressive or crushing pressure, under which the tack of the internal cell walls of the polyurethane composition adhere and retain the backing layer at its regauged height or thickness permanently, and with a high degree of uniformity following completion of the cure cycle. In the compressive regauging of the polyurethane backing prior to its complete curing, either an embossed or a smooth external surface is imparted to the layer with high uniformity. A temporary or a permanent surface film or skin is or may be also produced on the cured cushion backing layer by the application of the detackifying agent or agents which are applied liberally to the layer and to the regauging means in order to prevent adherence of the incompletely cured material to the regauging equipment.
Other important features and advantages of the invention will become apparent during the course of the following description.
In both the patented and commercial prior art, a number of teachings pertaining to the direct application of reactive polyurethane compositions to the primary carpet backing have been set forth. One pertinent prior art teaching is contained in U.S. Pat. No. 4,132,817. This patent discloses a method of applying a blown polyurethane backing layer to a primary carpet backing. A reactive polyurethane mixture is applied to a conveyor belt on top of a pre-applied latex film on the belt. Blowing of the reactive mixture takes place under a controlled temperature. A preheated carpet web has its primary backing brought into surface-to-surface contact with the polyurethane material undergoing blowing, and pressure is applied to the carpet web for the purpose of shaping the polyurethane layer and to promote bonding thereof to the primary backing of the carpet. At the completion of curing, the united carpet web and cellular polyurethane backing layer are stripped from the conveyor belt. The apparatus employed in this patented process is rather complicated and excessively costly. The reactive polyurethane composition is not applied directly to the primary carpet backing but must be transferred from a supporting belt to the primary backing during the shaping operation which is extremely difficult to control because of the nature of the blowing process. The end result obtained by the patented process, in addition to being more costly, falls far short of the present invention in terms of quality and precision uniformity in the applied cushion backing.
A commercial prior art process, namely the ICI Polyurethane Carpet Backing Process of Rubicon ICI in West Germany, involves direct application of a reactive polyurethane mixture to the primary backing of carpet by means of traversing spray nozzles which are fed from stationary mixing heads. The moving carpet is preheated by infrared heaters. After blowing of the applied polyurethane layer, curing takes place within an oven having infrared heaters. Approximately five meters downstream from the curing oven, the product is subjected to the action of a heated embossing roller which produces a light embossment in the backing layer and improves the appearance of the product. The ICI commercial process fails to recognize that embossing of the polyurethane backing layer after the curing cycle or very late in such cycle renders it impossible to achieve the main advantages of the present invention in terms of surface smoothness and uniform thickness of the applied cushion backing regardless of variations in thickness of the backing during application because the walls of the internal cells of the blown backing have already lost their tackiness at the time they are acted on by the light embossing roller in the ICI process.
None of the known prior art suggests the essence of the invention set forth above, wherein the reactive polyurethane mixture may be applied evenly or somewhat unevenly to the primary backing of the carpet and subsequently regauged to a precision uniform thickness early in the cure cycle while the internal cell walls are still substantially tacky and adherent.
Additional prior art U.S. patents of general interest and which do not suggest the discussed novel features of the present invention are the following, which are made of record herein under 37 C.F.R. 1.56:
______________________________________ 2,758,036 3,519,526 2,939,851 3,811,922 3,175,936 3,819,463 3,296,009 3,926,700 3,314,845 4,022,643 3,505,252 4,035,529 3,518,102 4,132,817. ______________________________________